1. Field of the Invention
The present invention relates to aluminum alloys suitable for use as machine or construction materials, and more particularly to an Al-Zn-Mg alloy having excellent properties, such as extrusibility, malleability and forgeability, which are essential as machine and construction materials.
In this specification the alloy contents are indicated in terms of per cent by weight.
2. Description of the Prior Art
Of the AA7000 alloys, namely aluminum-zinc-magnesium alloys, the 7003 alloy is known for its strength, extrusibility and forgeability. Of a variety of aluminum alloys including the 7000 alloys the 7075 alloy is well known for its strength and malleability. Nevertheless, the 7003 alloys lack the extrusibility, and the 7075 alloys lack the malleability for practical purposes.
Recently every industrial field requires thin, light-weight sheet materials. In order to enhance the strength of alloys without trading off their extrusibility and malleability, the common practice is to add more zinc or magnesium. However, the addition of zinc makes the alloy susceptible to stress and corrosion. As a result such alloys become unsuitable for construction.
An excessive amount of magnesium tends to impair the malleability, and make it hard, thereby reducing the workability of the alloy. The 7075 alloy per se is susceptible to stress and corrosion, and therefore it is necessary to heat it to a higher temperature, and then temper it for a longer period of time than for T.sub.6 -alloy, so as to stabilize the structure and attain as tempered a state as the T.sub.7 -alloys. Owing to this special heat treatment the strength is unavoidably sacrified by 10 to 20%.
After all it is difficult to obtain aluminum alloys having sufficient strength, resistance to stress and corrosion, and being excellent in extrusibility, forgeability and workability.